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Nephrons
8
...
8
...
The beginning of the tubule lies in the renal cortex and is expanded into a cupshaped capsule – Bowman’s capsule – which encloses a small network of
capillaries called the glomerulus
...
Capillaries surround the tubule until it joins the collecting duct which leads to
the renal pelvis (centre) of the kidney (k Box 8
...
Box 8
...
The kidneys retain their capacity
to maintain normal function
except at times of stress, e
...
infection
...
8
...
Box 8
...
The ultrafiltrate is therefore devoid of
proteins, blood cells or platelets
...
Less than
1% of the water and solutes in filtrate
are excreted because of reabsorption
by other segments of the nephron
...
Box 8
...
This is the
reason why urine should not normally
contain any significant amounts of
protein
...
afferent arteriole
efferent arteriole
Functions of the nephrons
As blood passes through the kidneys, three key processes take place in the nephrons
that result in urine:
• ultrafiltration of the blood in the glomerulus that filters out urea, salt, water,
glucose and other small particles from the blood (k Box 8
...
g
...
8
...
1 Ultrafiltration
Ultrafiltration (glomerular filtration) occurs in the glomerular capillaries, which
act as a semipermeable membrane (k Box 8
...
The efferent arteriole carrying
blood away from Bowman’s capsule has a smaller diameter than the afferent
arteriole, making the resistance in the efferent arteriole higher than that in the
afferent arteriole (k Fig
...
4)
...
g
...
The fluid filtered in this way is called glomerular
filtrate
...
• Hydrostatic pressure in glomerular capillaries tends to push water and solutes
into Bowman’s capsule (k Fig
...
4: red arrows)
• Forces that oppose glomerular filtration are:
– colloid osmotic pressure of blood which tends to attract fluid back into blood
(k Fig
...
4: blue arrows)
– hydrostatic pressure of the filtrate already in Bowman’s capsule (k Fig
...
4:
yellow arrows)
...
There is a natural decline in glomerular filtration as people get
older
...
2
...
8
...
Reabsorption allows many useful solutes that have been filtered out from the blood
in Bowman’s capsule, to return to the blood in different segments of the nephron (k
Fig
...
5)
...
8
...
Á The descending limb of the loop of Henle is permeable to water but
impermeable to sodium chloride
...
 The ascending limb of the loop of Henle actively transports sodium chloride
back into the blood
...
Overall, relatively more sodium chloride than
water has been reabsorbed by the loop, making the filtrate very dilute when it
enters the distal convoluted tubule
...
100
4
distal
convoluted
tubule
300
CORTEX
MEDULLA
300
300
1
proximal
convoluted
tubule
2
descending
limb of the
loop of Henle
100
Blood from
efferent
arteriole
DCT and
collecting
ducts
NaCl
200
H2O
ADH regulates
permeability
400
NaCl
200
H2 O
H2O
3
ascending
limb of the
loop of Henle
ascending
limb of
vasa recta
descending
limb of vasa
recta
400
H2 O
urea
NaCl
H2 O
900
700
600
NaCl
urea
1200
Vasa recta
1200
Fig
...
5 Variation in reabsorption of solutes from filtrate in different segments of
the nephron
...
6)
...
8
...
The luminal membrane is selectively permeable and determines which
substances from the filtrate are reabsorbed and by what method:
• sodium ions (Na+) are actively transported (k Fig
...
3), a process which requires
energy provided by ATP
...
5
...
2
...
Box 8
...
g
...
The epithelium of the proximal tubule and loop of Henle is impermeable to the
nitrogenous waste molecule urea so it becomes increasingly more concentrated as
it flows along towards the distal tubule (k